Red beds (or redbeds) are sedimentary rocks, which typically consist of sandstone, siltstone,

and shale that are predominantly red in color due to the presence of ferric oxides. Frequently, these

red-colored sedimentary strata locally contain thin beds of conglomerate,marl, limestone, or some

combination of these sedimentary rocks. The ferric oxides, which are responsible for the red color of

red beds, typically occur as a coating on the grains of sediments comprising red beds. Classic

examples of red beds are the Permian and Triassicstrata of the western United States and

the Devonian Old Red Sandstone facies of Europe.[1][2]

Contents

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1 Primary red beds

2 Diagenetic red beds

3 Secondary red beds

4 See also

5 References

6 External links

Primary red beds[edit]

Krynine (1950) suggested that the red beds were primarily formed by the erosion and redeposition of

red soils or older red beds. A fundamental problem with this hypothesis is the relative scarcity of

Permian red colored source sediments to the south of Cheshire. Van Houten (1961) developed the

idea to include the in situ (early diagenetic) reddening of the sediment by the dehydration of brown

or drab colored ferric hydroxides. These ferric hydroxides commonly include goethite (FeO-OH) and

so called "amorphous ferric hydroxide" or limonite. In fact, much of this material may be the

mineral ferrihydrite (Fe2O3 H2O).

This dehydration or "aging" process is now known to be intimately associated

with pedogenesis in alluvial floodplainsand desert environments. Berner (1969) showed

that goethite (ferric hydroxide) is normally unstable relative tohematite and in the absence of water

or at elevated temperature will readily dehydrate according to the reaction:

2FeOOH (goethite)→ Fe2O3 (hematite) +H2O

The Gibbs Free Energy for the reaction goethite → hematite (at 250 °C) is -2.76kJ/mol and

Langmuir (1971) showed that G becomes increasingly negative with smaller particle size. Thus

detrital ferric hydroxides including goethite and ferrihydrite will spontaneously transform into red

colored hematite pigment with time. This process not only accounts for the progressive

reddening of alluvium but also the fact older desert dune sands are more intensely reddened

than their younger equivalents.

Diagenetic red beds[edit]

The formation of red beds during burial diagenesis was clearly described by Walker (1967) and

Walker et al. (1978). The key to this mechanism is the intrastratal alteration

offerromagnesian silicates by oxygenated groundwaters during burial. Walker’s studies show

that the hydrolysis of hornblende and other iron-bearing detritus follows Goldich dissolution

series. This is controlled by the Gibbs Free Energy of the particular reaction. For example, the

most easily altered material would be olivine: e.g.

Fe2SiO4 (fayalite) + O2 → Fe2O3 (hematite) + SiO2 (quartz) with E = -27.53kJ/mol

A key feature of this process, and exemplified by the reaction, is the production of a suite of

by products which are precipitated as authigenic phases. These include mixed layer clays

(illite – montmorillonite), quartz, potassium feldspar and carbonates as well as the

pigmentary ferric oxides. Reddening progresses as the diagenetic alteration becomes more

advanced and is thus a time dependent mechanism. The other implication is that reddening

of this type is not specific to a particular depositional environment. However, the favourable

conditions for diagenetic red bed formation i.e. positive Eh and neutral-alkaline pH are most

commonly found in hot, semi-arid areas, and this is why red beds are traditionally

associated with such climates.

Secondary red beds[edit]

Secondary red beds are characterized by irregular color zonation, often related to sub-

unconformity weathering profiles. The color boundaries may cross-cut lithological contacts

and show more intense reddening adjacent to unconformities. Johnson et al. (1997) have

also showed how secondary reddening phases might be superimposed on earlier formed

primary red beds in the Carboniferous of the southern North Sea. The general conditions

leading to post-diagenetic alteration have been described by Mücke (1994). Important

reactions include pyrite oxidation:

3O2 + 4FeS2→ Fe2O3 (hematite) + 8S E = -789 kJ/mol

and siderite oxidation:

O2 + 4FeCO3 → 2Fe2O3 (hematite) + 4CO2 E = –346 kJ/mol

Secondary red beds formed in this way are an excellent example of telodiagenesis.

They are linked to the uplift, erosion and surface weathering of previously deposited

sediments and require conditions similar to primary and diagenetic red beds for their

formation.